LHS (hydraulic fluid)

Citroën

Industry	Automotive
Founded	June 5, 1919 (1919-06-05) in France
Founder	André Citroën
Area served	Worldwide
Key people	Paul Magès (developer of hydropneumatic suspension)
Products	Automobiles, hydropneumatic suspension systems
Website	www.citroen.com

Hydropneumatic suspension is an automotive suspension technology developed by Paul Magès, manufactured by Citroën, and installed in Citroën vehicles. This system has also been licensed to various other automakers and is extensively utilized in contemporary tanks and military machinery. Initially, it was referred to as Suspension oléopneumatique, emphasizing its combination of oil and air components.

The technology provides a highly responsive and adaptable suspension with substantial load-bearing capabilities, merging hydraulic power transmission with pneumatic gas cushioning to ensure exceptional ride comfort over diverse terrain conditions. It usually features automatic leveling and adjustable ride height controlled by the driver, improving ground clearance.

Originating from aircraft pneumatic landing gear systems partially filled with oil for lubrication and gas retention—a concept patented in 1933—the hydropneumatic principles have since been widely adopted, including in aircraft oleo struts and gas-charged automotive shock absorbers.

Description

Conceived by Paul Magès and produced by Citroën, hydropneumatic suspension combines oil and air elements to enhance vehicle dynamics. Beyond Citroën, this technology has been licensed to companies such as Rolls-Royce (notably on the Silver Shadow), BMW (5-Series E34 Touring), Maserati (Quattroporte II), and Peugeot. It found applications in Berliet trucks and influenced the Active Body Control system by Mercedes-Benz. The 1991 Toyota Soarer UZZ32 "Limited" model incorporated a fully integrated four-wheel steering system alongside computer-regulated hydraulic Toyota Active Control Suspension. Comparable technologies are common in modern military vehicles, enhancing operational capabilities and occupant comfort.

Effects

Designed to provide a finely tuned, dynamic suspension with high load capacity, the system delivers excellent ride quality across various surfaces. It commonly includes both automatic leveling and driver-controlled ride height adjustment, facilitating improved clearance on uneven terrain. Hydropneumatic suspension offers distinct benefits over traditional steel springs by integrating two core technologies:

• Hydraulic mechanisms that multiply torque without relying on mechanical gears or levers.
• Pneumatic components employing compressible gas to absorb shocks more effectively.

Although frequently associated with enhanced comfort, hydropneumatic systems also improve vehicle handling and control, overcoming limitations found in conventional steel spring setups. Despite these advantages, early implementation encountered obstacles due to patent restrictions and perceived system complexity; manufacturers such as Mercedes-Benz, British Leyland (Hydrolastic, Hydragas), and Lincoln opted for simpler compressed air-based suspensions.

Maintenance of Citroën’s hydropneumatic vehicles required specialized equipment and expertise, setting them apart from standard automobiles. The poor road infrastructure in post-war France underscored the system’s ability to provide smooth, stable rides on models like the Citroën ID/DS.

While inherently lacking roll stiffness, hydropneumatic suspensions have seen significant performance improvements through innovations such as steel anti-roll bars, variable firmness settings (Hydractive), and active body roll control systems (Citroën Activa) over subsequent years.

Basic mechanical layout

This arrangement employs a pump driven by the engine's belt or camshaft to pressurize a specialized hydraulic fluid, which then powers the brake system, suspension, and power steering. Additionally, it can control components such as the clutch, turning headlamps, and power windows.

Nitrogen is selected as the compressed gas because it minimizes corrosion risks. The nitrogen gas reservoir functions as a spring and is activated by incompressible hydraulic fluid within a suspension cylinder. Suspension leveling is managed by modifying the fluid quantity inside the cylinder. Inside the suspension sphere, a rubber diaphragm separates the nitrogen gas from the hydraulic oil.

History

Citroën first implemented this technology in 1954 on the rear suspension of the Citroën Traction Avant. A year later, in 1955, the system was extended to all four wheels on the innovative DS. The concept was inspired by pneumatic suspensions used in aircraft, which Citroën had patented back in 1933. The design was further enhanced with the 1960 patent for the "Double stage oleo-pneumatic shock absorber".

Significant developments include:

• During World War II, Paul Magès, an employee at Citroën without formal engineering credentials, pioneered an oil-and-air suspension system that combined comfort, control, and self-leveling capabilities.
• 1954: The rear suspension of the Citroën Traction Avant 15H utilized LHS hydraulic fluid.
• 1955: The Citroën DS launched with hydraulic systems powering suspension, power steering, brakes, and clutch/gearbox via a belt-driven pump featuring seven pistons.
• 1960: US patent granted for a double-stage oleo-pneumatic shock absorber, which became foundational for aircraft oleo struts and gas-filled shock absorbers.
• 1965: Rolls-Royce acquired a license to use Citroën’s hydropneumatic suspension technology for its Silver Shadow model.
• 1967: Introduction of non-hygroscopic mineral hydraulic fluid known as LHM.
• 1969: The Citroën M35 coupé, equipped with a Wankel engine and hydropneumatic suspension, was manufactured by Heuliez.
• 1969: Legal approval in the US for using LHM mineral hydraulic fluid.
• 1970: Hydropneumatic suspension adapted for smaller vehicles such as the Citroën GS.
• 1970: Debut of Citroën SM featuring DIRAVI variable-speed auto-returning power steering and hydraulically operated directional high beams via separate hydraulic circuits.
• 1972–1990s: BMW offered hydropneumatic rear suspensions as options on several 5-, 6-, and 7-series models; earlier versions used independent hydraulic systems, later integrated with power steering pumps.
• 1974: The US prohibited vehicles with adjustable height suspensions; this ban was lifted in 1981.
• 1974: The Citroën CX combined hydro-pneumatic self-leveling suspension with DIRAVI power steering and flexible subframe mounts to enhance ride quality and reduce noise.
• 1974: Maserati Quattroporte II utilized hydropneumatic suspension on an extended Citroën SM chassis during Citroën’s ownership period.
• Mid-1970s to 1980s: Mercedes-Benz incorporated hydropneumatic suspension in models like the 450SEL 6.9 and W126 S-Class variants, blending hydraulic height adjustment with coil springs.
• Late 1980s–1990s: BMW E30 Touring and E32/E34 series employed shared pump hydraulic setups to provide self-leveling rear suspensions.
• 1989: Introduction of Hydractive Suspension on the Citroën XM, featuring electronic control based on sensor input.
• Early 1990s: Peugeot implemented rear hydropneumatic suspension on the Mi16x4; JCB Fastrac tractors adopted this technology for high-speed agricultural applications.
• 1991: Toyota Soarer UZZ32 incorporated sensor-regulated hydraulic struts enabling dynamic suspension adjustments.
• Mid-1990s: The Citroën Xantia featured Hydractive systems along with optional Activa active suspension that counteracted body roll through active anti-roll bars.
• Late 1990s–early 2000s: Mercedes-Benz launched electronically controlled Active Body Control on CL-Class and S-Class models.
• Early 2000s: The Citroën C5 introduced Hydractive 3 technology combining pump and sphere units with electric height sensors; Hydractive 3+ further enhanced this system on C5 and C6 models.
• From mid-2000s onward: JCB Fastrac tractors upgraded to front and rear hydropneumatic suspensions.
• 2019: Mercedes-Benz unveiled eActive Body Control on GLE-Class SUVs, eliminating mechanical roll bars to improve handling.
• 2023: BYD Auto launched advanced active hydropneumatic suspensions on its Yangwang U8 SUV and U9 sports car, enabling unique features like three-wheel drive and stationary jumping while maintaining level stance.

Functionality

At the heart of the system lie the spheres—typically numbering five or six—which act as both pressure reservoirs and suspension components: one assigned to each wheel, a primary accumulator, and on certain versions, a brake accumulator. Advanced Hydractive or Activa suspension systems may incorporate up to ten spheres. Each sphere consists of a hollow metal shell containing a flexible Desmopan rubber diaphragm that separates it into two sections. The upper chamber is charged with high-pressure nitrogen gas (reaching up to 75 bar), while the lower chamber connects directly to the hydraulic fluid circuit. An engine-driven high-pressure pump pressurizes the LHM (liquide hydraulique minéral) fluid, sustaining system pressures ranging between 150 and 180 bars. This hydraulic energy primarily supplies the front brakes through a safety valve and can also operate steering, clutch mechanisms, or gear selectors depending on the vehicle configuration.

Suspension pistons are actuated by hydraulic pressure, which pushes fluid into the spheres, compressing the nitrogen gas and providing damping effects. A bi-directional 'leaf valve' regulates fluid movement, ensuring effective suspension damping. Self-leveling functionality is achieved through height corrector valves linked to anti-roll bars; these valves adjust fluid volumes in response to load variations, preserving consistent ride height while incorporating a deliberate delay to prevent reaction to routine suspension travel. Rear braking force is delivered from the rear suspension hydraulic circuit, with braking intensity modulated according to load.

Hydraulic Fluids Used

Initially, Citroën employed a red synthetic hydraulic fluid named LHS (Liquide Hydraulique Synthétique) from 1954 until 1967. However, this fluid absorbed moisture and airborne contaminants due to exposure within its vented reservoir, leading to corrosion problems. The reservoir’s design allowed humid air ingress as fluid levels fluctuated with suspension movements, deteriorating system performance over time.

To overcome these issues, Citroën introduced green mineral oil-based LHM (Liquide Hydraulique Minéral), bearing similarities to automatic transmission fluids. LHM’s hydrophobic nature prevents formation of water vapor bubbles and eliminates spongy brake pedal feel. Its corrosion inhibitors combined with an integrated reservoir filter reduce dust intrusion, significantly extending system durability. Adhering to proper maintenance routines—including filter cleaning and fluid replacement—is essential; using incompatible fluids risks damage and necessitates comprehensive flushing procedures.

Beyond Citroën’s use, mineral oil-based hydraulic fluids akin to LHM have been adopted by manufacturers such as Rolls-Royce, Peugeot, Mercedes-Benz, Jaguar, Audi, and BMW.

The most recent Citroën models equipped with Hydractive 3 suspensions utilize an orange-colored LDS hydraulic fluid that complies with DIN 51524-3 HVLP standards, offering enhanced longevity and extended service intervals.

Production Details

High-pressure components are fabricated from narrow-diameter steel tubing connected via Lockheed-style pipe unions sealed with Desmopan—a polyurethane thermoplastic compatible with LHM fluid. Dynamic parts like suspension struts and steering rams employ contact seals between cylinder and piston surfaces to preserve pressure integrity. Additional plastic or rubber elements include return tubes from valves such as brake control or height corrector assemblies; these tubes also capture any leaking fluid near suspension push-rod areas. Critical components including height corrector valves, brake master valve spools, steering valve spools, and hydraulic pump pistons maintain extremely tight tolerances (1–3 micrometres), minimizing leakage. While metal and alloy parts seldom fail even after extensive mileage, elastomeric parts exposed to air tend to harden and develop leaks—common points of failure.

Spheres themselves do not undergo mechanical wear but gradually lose internal nitrogen pressure due to gas diffusion through their membranes. Recharging spheres is more economical than replacing them entirely. Citroën’s Hydractive 3 suspension introduced spheres featuring nylon membranes that drastically reduce deflation rates; these can be recognized by their grey coloration.

Traditional green and grey spheres generally endure for 60,000 to 100,000 kilometers. Early sphere designs included threaded plugs for nitrogen recharging; newer "saucer" style spheres lack these plugs but can be retrofitted for gas recharge capability. The membrane lifespan is effectively unlimited unless subjected to low-pressure operation causing rupture. Regular recharging approximately every three years is vital for proper function. A ruptured membrane leads to loss of suspension at the affected wheel without altering overall ride height. Without spring action beyond tire compliance, driving over potholes with a deflated sphere risks damage to suspension components or wheel rims. Failure of the main accumulator sphere leaves only the high-pressure pump supplying front brake pressure; some older power steering models featured separate accumulators dedicated to front brakes.

Earlier LHS and LHS2 fluids (red in color) used elastomers incompatible with green LHM oil. Similarly, the orange LDS fluid utilized in Hydractive systems is not compatible with other hydraulic fluids.

Legacy

The foundational concepts of hydropneumatic suspension developed here have found extensive application, including in aircraft oleo struts and gas-charged automotive shock absorbers—initially patented in the United States in 1934 by Cleveland Pneumatic Tool Co.—as well as in contemporary armored vehicles such as tanks and other heavy military machinery.

Hydractive

Hydractive Suspension represents an automotive innovation launched by Citroën in 1990, building upon the 1954 hydropneumatic suspension system by integrating electronic sensors and driver-adjustable controls. First showcased on the 1988 Citroën Activa concept car, this technology enables drivers to toggle between a firm suspension setting (sport mode) and a softer, more comfortable ride (soft mode). Data from sensors monitoring steering, braking, suspension movement, throttle position, and gearbox status are transmitted to an onboard computer. This system then manages the activation or deactivation of supplementary suspension spheres to enhance either handling or ride comfort. The Activa variants exhibited a slight inward lean of one degree during cornering; however, Citroën determined that a neutral zero-degree lean was actually preferable.

From a commercial perspective, this system also contributes to lower fuel consumption and reduced tire degradation by minimizing tire scrub caused by negative camber during turns.

Hydractive 1 and Hydractive 2

The Hydractive suspension system by Citroën was implemented on models such as the XM and Xantia, including the technologically advanced Activa sub-model. Hydractive 1 provided two selectable driver modes: Sport, which maintained a consistently firm suspension setup, and Auto, which automatically alternated between soft and firm states based on sensor feedback.

Hydractive 2 enhanced this approach by introducing Sport and Normal modes. The Sport setting lowered sensor activation thresholds to ensure firm suspension during cornering and acceleration while preserving more ride comfort than its predecessor.

In cases where sensor readings were irregular—often due to electrical malfunctions—the system defaulted to a firm suspension mode as a safety measure.

Starting with the 1994 Xantia and the 1995 XM models, an additional sphere and valve assembly known as the SC/MAC sphere was incorporated. This acted as a hydraulic pressure reservoir for the rear brakes, enabling the vehicle to maintain proper rear ride height for extended periods without engine operation, thereby improving reliability and customer satisfaction.

Hydractive 3

Introduced with the 2001 Citroën C5, Hydractive 3 brought advancements in ride height regulation by preserving normal vehicle height even when the engine is switched off, facilitated by sophisticated electronic controls. This version utilizes orange synthetic LDS hydraulic fluid instead of the traditional green LHM mineral oil.

An enhanced variant termed Hydractive 3+ was launched on higher-spec C5 models and became standard equipment on the Citroën C6 starting in 2005. This system incorporates extra spheres that can be engaged via a Sport button to deliver a stiffer suspension feel.

Hydractive 3 features two automatic operating modes:

• Motorway mode: lowers vehicle ground clearance by 15 mm at speeds above 110 km/h
• Rough road mode: raises vehicle clearance by 13 mm at speeds below 70 km/h

Vehicle height adjustments are managed by the BHI controller based on speed as well as front and rear suspension heights.

Hydractive 3+ introduces a third setting:

• Comfort or dynamic mode, which varies suspension firmness dynamically

Its BHI unit processes additional inputs including steering wheel rotation and angle, longitudinal and lateral accelerations, suspension travel speed, and throttle activity to continuously optimize ride quality.

Model applications include:

C5 I (2001–2004)

• Hydractive 3: EW7J4, DW10TD engines
• Hydractive 3+: EW10J4, EW10D, ES9J4S, DW12TED4 engines

C5 II (2004–2007)

• Hydractive 3: EW7J4, EW10A, DV6TED4, DW10BTED4 engines
• Hydractive 3+: ES9A, DW12TED4 engines (prior to RPO No 10645)

C6 (2005–2012)

• Hydractive 3+: Standard across all trims

C5 III X7 (2007–2017)

• Hydractive 3+: Availability dependent on market and trim level

External links

• Comprehensive Citroën Technical Manual